Austin Whittall

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Friday, April 27, 2012

I read an online article[1] that prompted some interesting thoughts, its title: “Rise of Humans 2 Million Years Ago Doomed Large Carnivores”. The article is about a study presented a week ago at a workshop on climate change and human evolution at Columbia University's Lamont-Doherty Earth Observatory.

It seems that when our more distant relatives acquired the ability to make stone tools some 2 million years ago, our diet changed and this led to the demise of most carnivore species in Eastern Africa (from 29 species of large meat-eaters weighing more than 21.5 kg that existed before our appearance, only 6 survive nowadays).

Lars Werdlin has studied the matter and ruled out climate as a factor of disruption since only the large carnivores were hit, not the tinier ones. Climate would hit all alike. Furthermore, the decline in carnivore numbers began just when hominins started making stone tools and shifted their diet to include more meat, becoming less omnivorous. They were carrion eaters not hunters, but they successfully stole the kills of meat eating mammals, starving them.

Werdelin studied a time period between 3.5 and 1. 5 Mya (Million years ago) and found that not only did carnivores die out, but also omnivores that scavanged such as civets were gone too. They were in direct competition with the hominin carcass scavengers. Only “hypercarnivores” survived.

Werdelin (with Lewis) had already written about this in 2005 [2], and concluded: “ The extinction rate peaks at around 3.0 Mya after which it falls slightly, remaining nearly constant until 1.8 Mya, after which it increases considerably. The data support the hypothesis that the modern carnivoran guild of eastern Africa originated relatively recently, mostly within the last million years.”[2]. So we have extinctions in two distinct pulses, one about 1.8 Mya (when H. erectus made his Acheulean tools) which continues to this day and an older one 3 Mya.

Not all scientists agree with this, and point out that the correlation between the two events (rise of hominins and decline of carnivores) is not too strong.

American mammal extinctions and pre-sapiens hominids

What is most interesting is that the article is not talking about us, Homo sapiens it actually refers to our more distant Homo erectus ancestors, this made me wonder if a smiliar pattern could be traced in America before the arrival date of modern humans. If so, we could suppose that H. erectus or if later, the Neanderthals, were in America during an early period, devastating American mammals.

Since this is my first foray into this field, I will back up all my guesses with bibliography (see “Sources” below). I first decided to check the temporal boundaries of the different “Ages” (Land mammal ages or LMAs) mentioned in the studies. To my surprise these are, (allow me the pun) not set in stone. The cutoff of each LMA is variable. Nevertheless, below is the timeline we will work with and the names of the most important LMAs:

Uquian 2.5 to 1.5[5] or 1.2[3] Mya

Ensenadan 1.5 or 1.2 to 0.5[5] or 0.8[3] Mya

Lujanian 0.5 or 0.8 to 0.01 Mya

As you will see below, these in turn are subdivided into other “shorter” periods.

If we were to expect a pre-sapiens “into America” scenario, it would have to happen between 1.8 and 0.2 Mya, in other words the late Uquian, the Ensenadan or the early Lujanian. In Table 1, below (adapted from sources [3] and [4] ), I have shaded the time frame in red.

The hard facts

Table 1 Based on [3] and [4].

Table 1 shows us that genra have become extinct long before hominids even got out of Africa. It shows us that the rate of extinction is variable and that recently the “big” animals have been hit harder than the smaller ones (Lujanian vs. Ensenadan or Uquian), but, long ago, during the Chapadmalean, small and big alike disappeared and in large quantities. This probably reflects some other kind of event, maybe climatic or a combination of events that hit all-sized animals.

The event that marked the end of the Chapadmalean and start of the Uquian was the “Great American Biotic Interchange” about 2.5 Mya. It seems,[6] that: Large body mass and not food-niche was the main cause of extinction, since most herbivores were big, body mass caused them to go too. This paper’s appendix shows that few omnivores were hit.

Apparently this “Great American Biotic Interchange”, took place gradually: it was not a sudden invasion that wiped out the local endemic mammals [7].

Modern Humans and their impact in America

There is now doubt that no other contienent lost as many mammals in the Late Pleistocene as South America. [3] There is a clear spike in extinctions about 10,000 years ago during the Quaternary, and these are explained by two mainstream theories:[8]

Climate – Ecological changes. Pleistocene plants were more diverse and their growing season longer. Climate change (Pleistocene glaciations) wiped out many species putting stress on specialized herbivoeres and their carnivore predators.

Human hunting. Or “overkill” theory: humans irrupted into an isolated habitat with animals who could not recognize them as predators and adopt defensive attitudes –this explains why African megafauna survived: they co-evolved with humans and learned to adopt protective behaviours. And several authors (Patterson and Pascual, Webb and Marshall) uphold this theory.[3]

Regarding this overkill theory it is perhaps overblown. The extended use of fire and its destructive impact on the environement impacted negatively on local mammal species, perhaps even more than effective stone spears. Also: [4]

The archeological evidence indicates that the overhunting was focused on guanacos and deer, which, paradoxically, are those mammals that survived the extinction. The scarcity or infrequent occurrence of megammamals in archeological sites more likely implies that these mammals were less abundant in the area, not that they were ignored or inaccessible to humans...The extinction appears to have been more concentrated in taxa of South American origin. However, this is mainly apparent because many of the large mammals were xenarthrans (and litopterns and notoungulates). [4]

H. erectus or Neanderthal in America, what do extinctions tell us?

Its time to look at the possible impact our more distant relatives may have had in America. So lets take a look at mammal extinctions during the last four million years.

Below is Table 2 adapted from Fig. 6 in [9], which I colored to show when mammals were undegoing periods of growth (green) or extinction (red). Roughly, the Great Exchange took place between “C” and “B” (chapadmalean – Barrancalobean periods). During this period, extinctions are rife.

Table 2 Based on [9].

Another very similar Table, shown below (Table 3) is adapted from Alberdi et al (1993), who, show the extinction rate in South America during this period (Fig. 2 of [10]), and compare local endemic species with those coming from the Northern Hemisphere during the Great Exchange

This Table 3 replicates the trends of Table 2, (but here you get a time scale and not only the names of the different periods). The colors, once again represent extinctions (red) or growth (green):

Table 3 Based on [10].

Table 3 clearly shows that for autochthonous species, there are two periods of growth (in green), one ending about 4 Mya and the other between 1.2 and 0.3 Mya. Each growth period is followed by a period of decline, painted red, (4 – 1.2 Mya and 0.3 Mya until now). Newcomers from the north, are always increasing but, (gray shaded) in Recent times suffered a loss (when modern humans came on scene).

So we do have periods of extinction predating the arrival of modern humans in America, what about other hominids?

Probable indicators of ancient hominds in America

The time window for ancient hominids to enter America depends on when they left Africa. If , as I posted previously (First Asians were not Homo erectus), the “primitive” H. habilis left Africa and gave rise to the Damanisi people or Homo georgicus about 1.75 M years ago, they or the Damanisians could have arrived in America shortly after (i.e. 1.6 Mya). The same could apply for H. erectus whose remains in Asia date back to about 1.7 Mya.

What impact could they have had on the South American mammalian fauna? What do Tables 2 and 3 tell us?

Any impact they may have had was limited: The autochthonous species were in the midst of a long period of extinctions (probably due to the “Great Exchange”), whose pace was slowing down (less extinctions).

The hypothetical arrival of H. erectus 1.6 Mya would fall towards the end of this period, during the Vorohuan and Sanandresian periods (5+6 in Table 3 and V-S in Table 2). At that time there was an swift change and the local mammalians went into a period where more taxa was appearing than going extinct.

This period of growth is painted in green in both Table 2 and 3, and happened during the Ensenadan (7 in Table 3 and E in Table 2), Table 1 also shows this trend, as it indicates that extinction rate dropped to a puny 8% during the Ensenadan Age

Lack of extinctions between 1.2 and 0.3 Mya indicates lack of human predation (if we buy the theory that humans are the cause of extinction).

Could this indicate that, if they ever reached America 1.6 Mya, the H. erectus became extinct here about 1.2 Mya? Or were they here in such small numbers that their impact was minimal and all of the extinctions are due to natural causes?

The Neanderthal option. The second period of increase in extinctions begins about 300 kya (8 in Table 3 and L in Table 2). This is long before the appearance of modern H. sapiens in Africa, let alone America. So it could indicate that if these extinctions were due to human action, then the culprit were the Neanderthals, who had arrived in America and were using their refined hunting skills on the local mammalians.

Closing comments

Modern human activity impacts negatively on the environment and that is an undeniable fact. However the impact that a few thousand primitive pre-sapiens men armed with stone spears and fire could have on the global mammalian fauna is something that, in my opinion has to be proven.

What data can we glean from the extinction of Late Pleistocene fauna? The paucity of remains, the large “slices” (0.5 My) into which this period is split up in the articles that deal with this subject, and the differing opinions among experts in the field, leave plenty of room to doubt if we can identify among the “natural” noise, the signal of a band of H. erectus or Neanderthals killing off American mammals.

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